1. Field of the Invention
This invention is primarily concerned with a filter having elements with distributed constants which make use of at least two different types of coupling.
2. Description of the Prior Art
It is known to construct filters having distributed constants. A filter of this type is provided with resonators. The signal propagates by coupling between the consecutive resonators of the filter. Distributed-constant filters are fabricated by making use of the stripline technology, the resonators being deposited by metallization on one face of a low-loss dielectric whilst the metallization of the second face constitutes the ground plane.
It is also known to construct so-called combline filters having straight resonators, the ends of which are connected to the ground plane on the one hand directly and on the other hand through a variable capacitor.
Filters of the combline type present difficulties in regard to construction and achievement of the desired filtering action.
The proximity of the variable capacitors gives rise to problems of available space for the construction of the filter.
Moreover, it was believed prior to conception of the present invention that the fact of adding lumped-constant elements in a distributed-constant filter had the effect of increasing the bulk of this latter.
As disclosed in the article entitled "Narrow-band stripline or microstrip filters with transmission zeros at real and imaginary frequencies" by Kari T. Jokela, published in "IEEE transactions on microwave theory and techniques", vol. MTT-28, No. 6, June 1980, it is also known to construct bandpass filters having very high attenuations at the ends of the pass-band. The filters described in this article have an even number of distributed-constant resonators in which resonators placed symmetrically with respect to the center of the filter are coupled.
The filters in accordance with the present invention have distributed-constant resonators. The signal propagates by coupling between the constituent resonators of the filter. As will be explained hereinafter, the filters in accordance with the present invention have at least two types of coupling between successive resonators.
In the case of filters which make use of U-shaped resonators (also designated as hairpin resonators), the reversal of certain U-shaped resonators with respect to the arrangement of a filter of conventional type permits easy connection of a transversal coupler between resonators disposed symmetrically with respect to the center of the filter, for example in order to construct a filter having two very substantial attenuation zones which are symmetrical with respect to the center frequency of the filter. Filters of this type can be employed for example in order to form envelopes or in order to remove frequency side lobes from an electric signal.
When two types of coupling of a combline filter are employed, the variable-capacitance capacitors are spaced at a greater distance. This has the effect of achieving on the one hand a reduction in capacitive coupling between capacitors and on the other hand easier implantation of said variable-capacitance capacitors in the filters according as these capacitors are located at a greater distance from each other.